Cardiovascular disease is effecting an ever increasing proportion of the human population. Diseases of the vascular system may occur as a result of several etiologies which lead to the development of atherosclerosis. Atherosclerotic vascular disease (hardening of the arteries) occurs in two predominant manifestations. In one form, a narrowing of blood vessels impedes blood flow in one or more regions within the vessel lumen. In another form, arterial wall degeneration with a formation of aneurysms causes the wall of the affected artery to weaken and balloon outward by thinning. Management of both narrowed and dilated arteries in the peripheral circulation has come under the domain of vascular surgery.
Investigators in the field of vascular surgery had at one time handled both narrowed and dilated arteries by a technique for suture anastomosis (sewing together) of two arterial segments using needle and thread. A significant advance in the field of vascular surgery, however, was the development of the technique of vascular bypass grafting. Vascular bypass grafting involves the use of either accessory vessel segments or artificial arteries to bridge defects in the vascular system, or to correct narrowing or blockages in affected arteries. This was a significant advance because prior to the implementation of this technique, there existed no significant therapeutic treatment for an individual having an abdominal aortic aneurism, that is, a dilatation of the main artery of the body. Such a patient had to live with the threat of aortic aneurysm rupture and death.
One clinical approach known to vascular surgeons for patients having a large aneurysm in their abdominal aorta is an abdominal aortic aneurysm repair. This repair operation involves a long, abdominal incision extending from the lower border of the breast bone down to the pubic bone so that the abdominal aorta and the aneurysm can be exposed and a prosthetic arterial graft can be implanted. The operation requires a general anesthesia with a breathing tube, extensive intensive care unit monitoring in the immediate post-operative period, along with blood transfusions and stomach and bladder tubes. The operation itself takes approximately six hours and, if no complications occur, the patient can return home within seven to fourteen days after convalescence.
Today, there is a significantly less invasive clinical approach known as endovascular grafting. This procedure involves the use of prosthetic or other vascular grafts in combination with vascular stents, so called "graftstent" combinations. J. C. Parodi et al., Transfemoral Intraluminal Graft Implantation for Abdominal Aortic Aneurysms, 5 ANNALS OF VASCULAR SURGERY 491 (1991). Stents are devices which permit fixation of a graft to an arterial wall without sewing. Conventionally, a stent-graft-stent (" graftstent") complex is loaded onto a catheter which is then guided through a body lumen to the general vicinity of an aneurysm. An angioplasty balloon expands the first stent until it bears firmly against the arterial wall, the balloon is then deflated and the catheter is withdrawn so that it can be reloaded or replaced. The catheter is then guided to the second stent so that the second stent can be expanded.
The stent which has had the greatest experimental and clinical application for endovascular surgery is the balloon-expandable Palmaz stent. U.S. Pat. No. 4,776,337. This stent can be reliably and readily affixed to any graft material currently employed for bypass. The Palmaz stent is made of a multiply slotted tubular piece of stainless steel. The slots allow the stent to maintain a small overall diameter prior to use for easy introduction to the appropriate artery and location within the body. Once the stent has reached that location, it may be expanded to the desired diameter using a conventional angioplasty balloon to form a tight friction seal to the artery wall.
Investigators have noted encouraging preliminary results with the endovascular grafting technique; nevertheless, a number of issues have been encountered during the procedure. A significant issue with respect to endovascular grafting techniques concerns the potential shift in location of a graftstent device that is deployed with a conventional angioplasty balloon. Theoretically, a balloon deployed stent can be deployed exactly at the desired site. In fact, a suitably positioned stent can shift, rotate or separate from an angioplasty balloon during the insertion or inflation of the balloon which lends a level of uncertainty to the procedure.
A separate source of concern to surgeons using the Palmaz stent with conventional deployment devices is that the metallic struts of the stent itself can perforate the angioplasty balloon during loading of the graftstent, advancing the graftstent to a designated site within a patient, or deploying the graftstent. Any perforation in the angioplasty balloon will result in inadequate inflation of the balloon which results in either improper deployment of the stent or the need to restart or protract the process. It is the risk of perforation that necessitates the use of hydraulic fluids for filling the angioplasty balloon; if the balloon were filled with air and inadvertently perforated, the risk of embolism would be severe.
Perhaps the most significant issue concerns the rate of deployment of a graftstent complex, especially those larger diameter prostheses for use in the abdominal aortic artery. The rate at which the balloon can be inflated is limited by the size of the catheter inflation lumen and the speed with which fluid can be injected through the catheter and into the angioplasty balloon. The rate at which the balloon can be deflated is similarly impacted.
The rate of inflation and deflation of the angioplasty balloon bears directly on the stress induced on the heart during deployment because the intraluminal fluid pressure upstream of the balloon is increased at all times that the balloon blocks the artery, that is, whenever the balloon is not deflated. This increased fluid pressure occurs in beats, from the heart, which may cause the angioplasty balloon and stent to shift to a position more distal than the surgeon had intended to deploy the stent. In addition, when the balloon is inflated within the abdominal aortic artery, there is a sudden increase in pressure in the artery which could result in heart disfunction. Accordingly, there exists a need to provide more reliable and rapid deployment of graftstents as well as rapid recoil of the deploying means so that blood flow is minimally impeded.